1. Field of the Invention
The present invention relates to key telephone systems of the type employing common control tone and voice signalling, and more particularly to a right-of-way circuit for determining the priority of different signalling sources in such a system.
2. Description of the Prior Art
In current day key telephone systems tone and voice signalling has become an accepted standard. In such systems electronic amplifiers for each telephone instrument are employed to receive, amplify and acoustically transmit audible tones and voice signals as used in the system. The wiring in such a system is arranged so that individual amplifiers can receive simultaneous audio signals from up to three different sources, i.e.: ringing for calls from a central office or PBX, ringing and voice signalling for intercom calls and voice signalling from a direct station selection (DSS) console. Systems of this type are like the 17A system manufacured by GTE Automatic Electric Incorporated, the COM KEY manufactured by Western Electric Company and the K76 system manufactured by International Telephone and Telegraph.
In systems of this type noted above, electronic amplifiers and associated transducers are incorporated in each telephone of the system to replace the conventional ringer and/or buzzer. Signal tones to indicate incoming central office or PBX or intercom calls and voice messages are transmitted to the amplifiers for the purposes of audible signalling. Because each amplifier can clearly process only one audio message at a time, it is usually necessary to arrange a system so tone or voice signals are sent to an amplifier one at a time. In the WE COM KEY and ITT K76 systems noted above the circuitry is arranged to provide only one voice signal per system at any particular moment. In both systems the intercom calling party can send a voice signal to any other intercom station in the system. In the COM KEY system a DSS console is provided for voice signalling of any amplifier in the system. In the K76 system the confusion of multiple signals is avoided by providing only one voice signalling method, viz., the intercom. The COM KEY system while seeming to provide two methods of voice signalling, through one amplifier (intercom and the DSS) in actuality causes the DSS to use the voice signalling circuitry of the intercom, and provides only one such circuit. Both the COM KEY and K76 systems will process only one voice message at a time as indicated, but tone signalling can occur during a voice signal, causing disruption of the voice message, in the K76. The present circuitry was designed for use in the GTE Automatic Electric's 17A system to permit multiple voice signalling while controlling all system signalling to avoid the possibility of one amplifier receiving tone and voice or two voice messages simultaneously. The present circuitry therefore providing a substantial improvement over the key telephone systems of the prior art.